There are electroacoustic components in which transducer structures convert between RF signals and acoustic waves. Such components generally comprise a piezoelectric material and electrode structures as part of the component structures. Such components can operate with surface acoustic waves (SAWs) or with guided bulk acoustic waves (GBAWs). Such components comprise a piezoelectric substrate, which can generally be present as a crystal.
Such components can be used in RF filters, e.g. in front-end circuits of mobile communication devices. If for example a bandpass filter operates with acoustic waves, then very steep bandpass edges in conjunction with small component dimensions are made possible. This is necessary on account of the close juxtaposition of different frequency bands.
What is problematic is that the position of the center frequencies and the positions of the filter edges may be dependent on temperature. The piezoelectric material generally expands at relatively high temperatures, such that the half wavelength Λ/2 defined by the finger spacing increases as the temperature rises. As a result, characteristic frequencies shift toward lower frequencies. A further general problem of electroacoustic components comprising a piezoelectric material is the value of the electroacoustic coupling coefficient κ2 (more precisely: the value of the magnitude |κ2|). In general, the highest possible values for κ2 are desired.
In addition to the temperature-induced linear expansion, the stiffness values, i.e. the entries of the stiffness tensor cijkl, may also change and thus lead to a further temperature dependence of the electrical properties.
One typical piezoelectric material for electroacoustic components is quartz, in which Rayleigh waves are capable of propagation. The electroacoustic coupling coefficient κ2 may be up to 0.132% in this case.
Electroacoustic components based on quartz as piezoelectric material are known e.g. from the documents EP 01679794 A2, WO 2007/037457 A1, U.S. 2007/194657, EP 01816744 A1, U.S. Pat. No. 6,946,930 B2 or from the document EP 2403141 A1.
A further typical piezoelectric material for electroacoustic components is lithium tantalate. Components based on lithium tantalate are known for example from the document U.S. 2003/0141947 A1 or the document U.S. 2003/0137367 A1.
However, a perfect piezoelectric material having a high electroacoustic coupling coefficient κ2 and a vanishing temperature dependence of the electrical properties is still not known.